The present invention relates to a liquid crystal display device. More specifically, the present invention relates to a driving method of a plain matrix panel using an SIN liquid crystal or the like. More specifically, the present invention relates to a driving method suitable for multiple line selection addressing.
The liquid crystal display device features compact size, light weight, flat shape and low power consumption, which are advantageous as compared to other types of display devices. Therefore, recently intensive work has been conducted for commercialization of the liquid crystal display device. The liquid crystal display device is generally classified an active matrix type and a plain or passive matrix type. The former type is constructed such that either a three-terminal element such as a thin film transistor or a two-terminal element such as an MIM diode is connected to each pixel to drive a liquid crystal. High contrast can be obtained compared to a static drive even through a number of multiplexing pixels increases. However, since the thin film semiconductor element is formed individually for each pixel, the construction is complicated to thereby raise production cost as the display size is expanded. On the other hand, the latter type is constructed such that rows of scanning electrodes and columns of signal electrodes sandwich therebetween a TN liquid crystal or an STN liquid crystal. Such a construction advantageously reduces a production cost. However, this type is driven in time-sharing manner according to a voltage averaging method, hence there is a drawback in that an effective voltage difference between ON and OFF states decreases as the multiplexing number is increased, thereby lowering the image contrast.
As the background, brief description is given to the voltage averaging method which is conventionally adopted for driving the plain matrix type liquid crystal display device. In this method, the respective scanning electrodes are sequentially selected one by one, while all of the signal electrodes are applied with data signals representative of the ON/OFF states of the pixels in synchronization with each selecting timing. Consequently, each pixel receives a high voltage of one time slot (1/N of a frame time interval) within one frame period during which N of the scanning electrodes are selected, while the same pixel receives a constant bias voltage in the remaining time interval ((N-1)/N of the frame time interval). In case that the liquid crystal material has a slow response, there can be obtained a brightness corresponding to an effective voltage of the applied waveform during one frame period. However, if a frame frequency is lowered as the multiplexing number increases, a difference between the one frame period time and a liquid crystal response time is reduced so that the liquid crystal responds to each applied pulse to thereby cause a brightness flicker called "frame response" which degrades the image contrast. FIG. 15 is a graph showing the frame response. A transmittance of the liquid crystal rises when a scanning electrode is selected, and then the transmittance gradually falls in a nonselecting period.
In order to eliminate the frame response using the voltage averaging method, two different countermeasures have been proposed, one of which is the "high frequency drive method" for reducing a width of a high voltage pulse, and the other of which is the "optimization of bias level" method for reducing a potential difference between the high voltage pulse and the bias voltage. FIG. 16 is a graph showing a transmittance variation in the high frequency drive. As compared to the FIG. 15 graph, the frame frequency is boosted as the pulse width is reduced. The high voltage pulse is applied at a selection timing by a shortened period, hence a next high voltage pulse is fed before the transmittance falls to a minimum level to thereby raise the overall transmittance. However, this high frequency drive has a drawback in that distortion of the applied waveform may seriously hinder uniformity of the displayed picture.
In turn, FIG. 17 is a graph showing a transmittance variation in case that the bias level is optimized. The bias voltage level is raised in the nonselection period so as to reduce an effective voltage difference between the selection and nonselection periods. As compared to the FIG. 15 graph, the fall of the transmittance is saved in the nonselection period. However, this bias level optimization method suffers from a drawback in that a voltage ratio of ON and OFF states decreases to degrade the display contrast.
With regard to the various drawbacks of the voltage averaging method, a consistent solution has been proposed "Multiple Line Selection", which was reported, for example, in SID '92 DIGEST pp232-235, 1992, by Optorex. Further, a similar method the "Active Addressing Method" was disclosed in SID '92 DIGEST pp228-231, 1992, by In Focus Systems, Inc. These multiple line selection methods are based on the principle of the high frequency drive; however, a multiple of lines are concurrently selected in contrast to the conventional single line selection to equivalently achieve the same effect as the high frequency drive. As opposed to the single line selection, the multiple line selection requires a specific technique for realizing a free display. Namely, an original picture signal is arithmetically processed to drive the signal electrodes. A basic computation scheme was proposed by T. N. Ruckmongathan in 1988 (1988 IDRC, pp80-85, 1988).
Further, In Focus Systems Inc. proposed "Pulse-Height Modulation (PHM) Gray Shading Methods for Passive Matrix" in JAPAN DISPLAY 1992-69, which can be combined with the multiple line selection method. In this pulse-height modulation gray shading method, a virtual scanning line is provided in addition to a plurality of actual scanning lines. A virtual picture data set is assigned to pixels on the virtual scanning line. This virtual data is computed based on picture data (dot data) which is assigned to actual pixels. On the other hand, a signal waveform applied to each signal line is obtained by arithmetically processing those of the actual and virtual picture data according to the aforementioned multiple line selection method. By providing the virtual line in such a manner, each pixel can receive a correct effective voltage according to the given picture data. Stated otherwise, the virtual line is provided for adjustment in order to correctly apply an effective voltage to the pixels according to the given picture data.